Braking surfaces, particularly those oriented on braking rotors, such as cast iron drums and disc brake rotors, have conventionally been tooled during primary manufacture by lathe cutting tools to present dimensioned machined surfaces having required tolerances. This manufacturing process produces in such braking rotors, in the form of brake discs or brake drums, a surface roughened with tooling marks and characterized by specific surface strength and hardness, a reformulated surface finish and tooling marks presenting a non-uniform stress profile and a source of tensile stress concentration. These tooling marks and tensile stresses limit the active service life of the braking surfaces thus encouraging fatigue cracks by heat checking and scuffing.
This problem has been addressed in U.S. Pat. No. 5,193,375, T. J. Meister, Mar. 16, 1993, entitled METHOD FOR ENHANCING THE WEAR PERFORMANCE AND LIFE CHARACTERISTICS OF A BRAKE DRUM and in U.S. Pat. Nos. 5,352,305, L. B. Hester, Oct. 4, 1994 and 5,664,648, L. B. Hester, Sep. 9, 1997, each entitled PRESTRESSED BRAKE DRUM OR ROTOR. These patents disclose a shot peening cold working method of treating the initially manufactured braking surface thereby inducing greater roughness in the braking surface. This peening creates a displacement of surface material about an impact point to establish a rim pattern of roughened peaks, to mask former tooling marks and tensile forces inherently introduced by the initial tooling procedure.
Objection can be taken to the roughened braking surface caused by peening. For example, such roughened braking surfaces require an intense burnishing process which develops heat checking when confronting the topography of brake pad surfaces thus encouraging heat checking hot spots which reduce the service life of the braking surface.
Furthermore, because of the nature of the product, particularly the confined and inaccessible interior braking surfaces of cast iron brake drums, it is physically difficult with peening methods to rework the braking surface areas without overlapping onto adjacent surfaces. Also precise and uniform control in the peening process to repeatedly meet different braking surface specifications is rarely feasible and practically impossible, thus leading to a low reliability of the peening method.
Proposed use of ultrasonic energy to restructure butt welded seams with an ultrasonic horn transducer directly in contact with the weld seam joining thin titanium sheets is taught by S. E. Jacke in U.S. Pat. No. 3,274,033, Sep. 20, 1966 for ULTRASONICS. However, the disclosed method and equipment would not be operable for processing the heavy cast iron structure used in brake drums and rotors, nor would it be able to process the sub-surface material to the depths needed for meeting braking surface specifications.
Furthermore, the ultrasonic treatment of welds requires a completely different relationship of ultrasonic impacts on the work body surface to strengthen welds.
Rather, the processing of braking surfaces to achieve improvements of braking surface strength for the compressive interfacing between the braking surface and braking pads affects the work product in an entirely different way.
Prior ultrasonic impact treatment methods have been utilized for treating weld joints and fractures in structural iron, but it has not been known how to treat the topography and strength of braking surfaces ultrasonically to obtain greater strength and ability to perform the in-service role of compressively confronting brake pads.
Prior art brake drums are in general subject to thermo-mechanical fatigue, and propensity to develop cracks. The braking surfaces between drums and brake pads do not have appropriate shape and texture for service and thus need be subjected to extensive break-in procedures which themselves tend to produce cracking and thermo-induced stresses. The peening method of brake surface treatment leaves a roughened braking surface that reduces the amount of effective area of the braking surface in use, produces uneven distribution of individual peened peaks and contact area sizes to thus have the propensity to develop hot spots, thereby seriously limiting the maximum braking moment and contributing to early fatigue failures.
Further significant limitations of the peening method include the depth of allowable wear on the working surface and the uniformity of braking characteristics during the life of the brake drum.
One further limitation of the peening method is the inability to control the process to meet variable specifications. For example, the only control over braking surface roughness is the complicated matter of finding and implementing the appropriate shot sizes and accompanying striking forces. There is essentially no way to achieve a smooth cylindrical interface fit with an associated brake pad with the peening method.
Background technology for the application of ultrasonic impact energy to the surface of polypropylene and thermoplastic materials for welding or riveting, is evidenced by U.S. Pat. No. 5,976,314, Nov. 2, 1999 by Manfred Sans for DEVICE FOR ULTRASONIC TREATMENT OF WORKPIECES. However, this teaching does not disclose a feasible system for the reworking of machined metal work pieces by ultrasonic machining methods in a manner reworking the sub-surface crystalline structure and establishing the surface texture of a metal workpiece, as does this present invention.
Similarly, various specialty ultrasonic metal working impact transducers, including hand operated tools, are disclosed in the prior art for surface deformation and subsurface plasticization of explicit shapes and contours, typically cylinders, rotating surface segments and planar sheets by direct mechanical interfacing of an ultrasonically vibrating head with a metallic surface work site. Typical disclosures are found in Russian inventor's certificates including: SU 1447646 A1, published Dec. 30, 1988; SU 1263510 A2, published Oct. 15, 1986; SU 1756125 A1, published Aug. 23, 1992; SU 1255405 A1, published Sep. 7, 1986; SU 1576283 A, published Jul. 7, 1990; SU 998104, published Jan. 5, 1981; SU 1214396 A, published Feb. 28, 1986; SU 1481044 A, published Sep. 28, 1987; and SU 1703417 A1, published Jan. 7, 1982 relating to direct mechanical contact between an oscillating ultrasonic transducer head oscillating at the prescribed ultrasonic frequency and the treated metallic object surface.
French 2,662,180 filed May 7, 1991 relates to a system for applying ultrasonically oscillating thermal impulse energy to induce plastic surface deformation at weld sites to improve the initial weld strength in the plastic material. This system does not disclose how to successfully apply ultrasonic energy to correct braking problems by machining brake drum surfaces.
Statnikov, et al published documents IIW XIII-1617-96 and IIW XIII-1609-95 relating to the state of the art of hand held tools for applying ultrasonic impact energy directly from an oscillating transducer head at the impacting resonant frequency of the driving oscillator. These transducers are special purpose transducers with a single impacting needle adapted to a system configuration for achieving the particular functional treatment of strengthening welded structure of a specific workpiece configuration at a specific periodic ultrasonic resonant frequency.
Thus, this state of the ultrasonic transducer prior art will permit one skilled in the art to select and apply appropriate ultrasonic transducers for practicing this invention.